Tell me please which tense I have to use in this context:
Kate: John, you are going to visit your grandparents this week, aren't you?
John: Actually, it is not set in stone yet. (OR: It has not been set in stone yet.)
I feel both options might be correct there, if I am right, then what is the difference?
The difference between is set in stone and has been set in stone is not temporal in nature, but rather the difference between simple state (is set) and state-as-result-of-agent-action (has been set).
The underlying verb of the latter is to be set, a passive infinitive.
Both options are correct and the difference is very small.
Present Simple only tells about now.
It is not set in stone.
Is it set now? No.
Was it set in the past? Unknown.
Will it be set in the future? Unknown.
Present Perfect tells about now AND that it started sometime in the past.
It has not been set in stone.
Is it set now? No.
Was it set in the past? No.
Will it be set in the future? Unknown.
(Notice that the present perfect answers what was unknown about the past.)
However, your examples include the word "yet".
That provides more information to the Present Simple statement.
Present Simple + "yet"
It is not set in stone yet.
Is it set now? No.
Was it set in the past? No.
Will it be set in the future? Unknown.
The difference is so small that a listener can understand or at least assume the same information from both of your options, but hopefully understanding the difference between present simple and present perfect is helpful.
